The invention relates to controlling a brake chopper and in particular to controlling a brake chopper of a DC voltage intermediate circuit of a frequency converter, and to a brake chopper and a frequency converter provided with one.
Brake choppers are used in the DC voltage intermediate circuits of frequency converters in a prior art manner for preventing an increase in the intermediate circuit voltage and for reducing the voltage when a device that normally fed from the frequency converter exceptionally feeds energy back to the intermediate circuit. This kind of situation arises for example when a magnetized motor rotates uncontrolled and thereby functions as a generator feeding power to the DC voltage intermediate circuit.
The situation is the same when a frequency converter is used for active braking of a rotating motor, whereby the power from the braking is transferred to the intermediate circuit. Drives in which braking is used often are usually equipped with a network braking supply unit that enables power to be transferred from the intermediate circuit through the supply unit into the feeding network. Then again drives in which braking is rather used on an occasional basis are typically provided with brake choppers, because a brake chopper is significantly more affordable to invest in than a network braking supply unit.
There are also cases in which an intermediate circuit of a frequency converter provided with a network braking supply unit nevertheless must be equipped with brake choppers. One example of this is isolated operation, such as the electric system of a vessel, where the braking energy generated in the motor may exceed other consumption in the electric system. In this situation the electric power network cannot be fed with more energy but it has to be removed by using a brake chopper in the intermediate circuit of the frequency converter.
A conventional method used for controlling brake choppers is tolerance band control, in which the intermediate circuit is provided with a predetermined voltage level and when this level is exceeded, a brake resistor is switched into the intermediate circuit through a chopper. The voltage in the intermediate circuit thus starts to decrease at a rate that depends on the braking power, the intermediate circuit capacitance and the resistance of the brake resistor. The resistor is kept switched on until the intermediate circuit voltage drops below the predetermined level, and the voltage starts to rise again.
Braking power can be increased by connecting a plural number of resistors in parallel. These parallel-connected resistors share the same control and therefore they also function simultaneously.
Publication U.S. Pat. No. 7,012,392 discloses a method and an apparatus for controlling a brake chopper. In this solution a plural number of voltage limits are determined for the intermediate circuit voltage. When the intermediate circuit voltage exceeds these limits or drops below them, the number of brake resistors switched on is increased or decreased in a predetermined order. This control method allows a fixed number of braking powers of different magnitudes to be obtained. In addition, it enables to reduce high intermediate circuit currents created in the conventional tolerance band control.
A problem with the above brake chopper is that the number of different braking powers to be obtained is fixed, which occasionally leads to an unnecessarily high braking power.